Shot Material and Shot Peening Method

ABSTRACT

A method of shot peening a workpiece comprising projecting metal alloy particles at a workpiece wherein said metal alloy particles comprises Fe in combination with B, C, Cr and Nb, wherein the Fe is present at a level of greater than 50.0 atomic percent. The metal alloy particles have a Vickers Hardness (HV) of at least 1150 and an elastic modulus of greater than 200 GPa.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/940,140 filed Feb. 14, 2014.

FIELD OF THE INVENTION

The present invention relates to a shot material for shot peening and ashot peening method and a treated article obtained by the process.

BACKGROUND

Shot peening is a mechanical surface treatment having the objective ofenhancing the resistance of mostly metallic components or workpiecesthat are subjected to cyclic loadings, wear, and corrosion and otherlife time reducing influences. During the shot peening process, theabrasive medium, also called shot medium, is propelled onto the surfaceof a workpiece. The impact of the medium dimples the surface of theworkpiece. The restoring force below the dimple in the workpiece resultsin a hemisphere of material that is relatively highly stressed incompression and a compressive residual stress field develops over thepeened surfaces as the dimples overlap during the peening process. Suchgenerated compressive stresses can provide a wide variety of benefits toa workpiece. For example, the useful life time of a workpiece may beincreased under cyclical loads or stresses caused by corrosion fromstress cracking, friction, cavitation, galling, erosion and wear, aswell as combinations of these kinds of stresses. It is commonly assumedthat these benefits are caused by the presence of the compressivestresses on the surface of the workpiece, since such stresses reduce thetendency of surface crack formation and or crack propagation.

Shot-peening media that have been reported include (1) carbides, carbidecomposites, or cermets (composite material composed of ceramic (cer) andmetallic (met) materials); (2) ceramics such as zirconia that arecommercially available for example as ceramic beads or ceramic shot suchas Microblast® B-120 or Zirblast® B-30 or B-400 from Saint Gobain.

Metallic based peening media are reported in U.S. Pat. No. 6,658,907. Aniron-based amorphous spherical particle is employed as the peeningmaterial that is said to preferably have an iron content of 45 to 55 wt.%, having a Vicker hardness (HV) in the range of 900-1100 and a Young'smodulus of 200,000 MPa or less.

U.S. Patent Application Publication No. 2011/0265535 discloses aniron-based shot peening material which comprises in mass % 5 to 8% of B,0.05-1% of C, 0 to 25% of Cr, balance of Fe and inevitable impurities,wherein B and C are contained in a total amount of 8.5% or less. Bcontents of less than 5% was described as providing insufficienthardness.

WO2009/133920A1 discloses an iron-based shot material made of B (5-8mass %), Al (10 mass % or less, preferably 0.5-10 mass %), Cr (0-25 mass%, preferably 1-25 mass %) and the remainder is Fe and unavoidableimpurities. The HV reportedly ranged from 1150-1300.

WO2012/128357 discloses a shot peening material containing, in mass %,2-8% of B and at least one element selected from Ti, Cr, Mo, W, Ni, Aland C in the amount fulfilling the formula: 0≦(Ti %/10)+(Cr %/25)+(Mo%/10)+(W %/6)+(Ni %/10)+(Al %/10)+(C %/1)≦1.00, and the remainder madeup by Fe and unavoidable impurities and having a particle diameter of 75μm or less.

SUMMARY

A method of shot peening a workpiece comprising projecting metal alloyparticles at a workpiece wherein said metal alloy particles comprises Fein combination with B, C, Cr and Nb, wherein the Fe is present at alevel of greater than 50.0 atomic percent wherein said metal alloy has aVickers Hardness (HV) of at least 1150 and an elastic modulus of greaterthan 200 GPa. The present invention also includes a workpiece that istreated by the above referenced method.

The present invention also relates to the shot peening material itselfcomprising metal alloy particles containing Fe in combination with B, C,Cr and Nb, wherein the Fe is present at a level of greater than 50.0atomic percent wherein said metal alloy has a Vickers Hardness (HV) ofat least 1150 and an elastic modulus of greater than 200 GPa.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description below may be better understood with referenceto the accompanying figures which are provided for illustrative purposesand are not to be considered as limiting any aspect of the invention.

FIG. 1 is a schematic illustration of the air-blasting cabinet forshot-peening.

FIG. 2 is a plot of comparative peening intensity saturation curves.

FIG. 3 is a plot of comparative residual stress fields for theidentified workpiece.

FIG. 4 is a plot of comparative workpiece hardness values.

FIG. 4A is plot of residual stress fields for the identified workpiece.

FIG. 4B is a plot of comparative workpiece hardness values.

FIG. 4C is a comparative plot of peening intensity saturation curves.

FIG. 4D is a plot of residual stress fields for the identifiedworkpiece.

FIG. 4E is a comparative plot of comparative workpiece hardness values.

FIG. 4F is a comparative plot of peening intensity saturation curves.

FIG. 5 is a comparative plot of peening intensity saturation curves.

FIG. 6 is a plot of comparative peening intensity saturation curves.

FIG. 7 is a plot of comparative workpiece hardness values at nearpeening intensity saturation.

FIG. 8 is a plot of comparative mass loss for the indicated particles.

FIG. 9 is a plot of comparative mass loss for the particles of thepresent invention at 1.0 hours and 6.0 hours versus Saint Gobain B120particles.

FIG. 10 is a comparative plot of mass loss.

FIG. 11 is a comparative plot of peening intensity saturation curves.

FIG. 12 is a comparative plot of peening intensity saturation curves.

FIG. 13 is a plot of comparative workpiece hardness values at nearpeening intensity saturation.

DETAILED DESCRIPTION

As noted above, the present invention relates to a shot peening mediathat provides relatively high hardness and durability. The alloy isgenerally understood as Fe based and includes B, C, Cr and Nb. Optionalelements include Mn, Si and V. Reference to Fe based may be understoodas the feature where the majority of the alloy composition comprisesiron (e.g., >50.0 atomic percent Fe). In addition, the alloy is one thatpreferably includes α-Fe (ferrite) and/or γ-Fe (austenite). The alloyalso preferably includes one or more of the following: (1) complexborides (e.g. M₁B, M₂B and M₃B where M is a transition metal); (2)complex carbides (e.g. M₁C, M₂C, M₃C, and M₂₃C₆ wherein M is atransition metal); (3) borocarbides (material containing both boride andcarbide atoms).

Preferably the alloy composition is comprised of the concentrationsidentified in Table 1 below:

TABLE 1 Preferred Alloy Composition Units Fe B C Cr Nb Mn Si Atomic %59.0-64.0 17.5-18.8 4.4-5.1 12.7-13.1 1.4-1.7 0-0.5 0-1.3 Weight %74.2-77.6  4.1-4.5  1.2-1.4 14.3-15.1 2.9-3.5 0-0.6 0-0.8

In addition to the above, the alloys herein may also preferably have thefollowing composition, in atomic percent: Fe (58.0-65.0); B (14.0-19.0);C (4.0-5.5); Cr (7.0-13.5); Mn (0-1.5); Nb (1.4-3.5); Si (0-1.5) and V(0-6.0).

The alloys herein are preferably prepared as particles by atomizationmethods. Exemplary atomization procedures include gas atomization,centrifugal atomization or water atomization. The particles may then besized using various techniques such as screening, classification and airclassification. Preferably, the particles are such that 95% of theparticles have a size range (largest linear dimension through theparticle) in the range of 40 μm to 250 μm. Accordingly, about 5% of theparticles may fall outside this range and indicate a particle sizedistribution in the range of 0.1-39.9 μm. More preferably, the D10 valuein microns (percent of the population having particle sized below thisnumber) is 50.0 μm. The preferred D50 value (median) is 80 μm and thepreferred D90 value (90 percent of the distribution below this number)is 150 μm.

In addition, the D10 value in microns may fall in the range of 50-100,the D50 value may fall in the range of 100-150 and the D90 value mayfall in the range of 150-200.

In connection with the particles noted above, preferably, the particleshave a spherical geometry. This may be understood as a shape having aset of points that are all the same distance r from a given point inthree-dimensional space. The value of r is the radius. Relatively lessspherical configurations that are also contemplated include relativelymore angular shapes that are referred to as grits.

The alloys are such that the particles indicate a HV value of at leastabout 1150. Preferably, the HV value may be in the range of 1150-1400.More preferably, the HV value is 1250+/−75. Accordingly, the HV valuemay preferably be in the range of 1175 to 1400. In addition, theparticles are such that they have an elastic modulus of greater than 200GPa, more preferably in the range of greater than 200 GPa up to 350 GPa.

Another feature of the particles herein is their associated durability.In that context the durability herein was characterized by projectingthe particles towards a workpiece under the following range ofconditions: projection pressure of 0.13 MPa to 0.82 MPa, a peeningvelocity of 80 m/s to 350 m/s and an Almen A intensity of 2-12 mils. Thedistance to the workpiece is in the range of 76-153 mm. The workpieceand media are then preconditioned for a period of 15 minutes prior totesting. The workpiece is a 6.35 mm thick steel alloy containing about13% manganese having a hardness of 697 Vickers. The metal particlesafter such projection on the workpiece for a period of 18 hours is suchthat, for a 100 gram portion of the particles, the weight fraction ofparticles of less than 75 μm that are present is less than or equal to7.0%.

The particles herein may be employed in two different types of equipmentdevices that are used to impart kinetic energy into the particles, suchas air nozzle-type systems or centrifugally wheel based system. Theexamples listed herein were conducted by an air nozzle system (FIG. 1).More specifically, a Kelco air blasting cabinet was employed with thefollowing parameters: nozzle type: venturi with an exit diameter of7.9375 mm; distance to workpiece of 101.6 mm; mass flow rate of 2.63kg/minute. However it is contemplated that all of the disclosed benefitswould be observed for wheel based systems. In addition, the particlesherein, while applied in the examples to workpieces having the indicatedcharacteristics, would be applicable to any workpiece where theadvantages of treating with impacting particles would be of benefit.

Working examples of the present invention, in comparison to variousshot-peening media, are supplied below. However, it may now beappreciated herein that the relatively high hardness and durableshot-peening media herein will be applicable for processes other thanshot peening, including, but not limited to de-sanding, de-scaling, andetching-prior-to-coating of any workpiece as well as for water jet orother cutting and sawing applications.

Example 1 & 1A

A metallic abrasive media of the invention (Example 1) was adopted as apeening media and tested as such. The spherical iron-based particle iscomprised of 12.7 at. % chromium, 18.8 at. % boron, 1.5 at. % niobium,4.6 at. % carbon, 0.3 at. % manganese, 0.8 at. % silicon, and theremainder (61.3 at. %) iron, having a specific gravity of 7.36 g/cm³, ahardness of 1284 Vickers. The particles had a particle size distributionof D10 equaling 52 micrometers, D50 equaling 83 micrometers, and D90equaling 142 micrometers.

Other metallic abrasive media of the invention were also prepared.Specifically, Example 1A is comprised of 7.77 at. % chromium, 14.73 at.% boron, 2.68 at. % Nb, 5.45 at. % C, 1.17 at. % Si, 4.68 at. % V and62.45 at. % iron. Example 1B has the same alloy composition as Example1.

For comparison, two commercial peening media were initially employed:(1) Sinto Microshot SBM-100C, having as specific gravity of 7.6 g/cm³, ahardness of 729 Vickers and a particle size distribution of D10 equaling100 micrometers, D50 of 132 micrometers, and D90 equaling 183micrometers; (2) Sinto AMO Beads AM-100, having as specific gravity of7.4 g/cm³, a Vickers hardness of 788 and a particle size distribution ofD10 equaling 64 micrometers, D50 of 92 micrometers, and D90 equaling 134micrometers.

The above referenced shot media particle hardness testing was performedusing a Tukon 2500 Knoop/Vickers Automated Hardness Tester (2000×optics) with a Vickers diamond indenter. Particle specimens wereprepared by add-mixing them with epoxy resin, curing, and then polishingthe mix to expose particle cross sections. A test load of 100 grams wasused, and the average of twelve indents reported, as shown below inTable 2:

TABLE 2 Shot Media HV Example 1 1284 Example 1A 1236 Example IB 1176Sinto Micro shot SBM-100C 729 Sinto AMO BEAD ™ AM-100 788 Saint GobainMicroblast ™ 692 B120

Particle size distribution was determined using a Microtrac 53500 laserdiffraction analyzer. The distributions are noted below in Table 3:

TABLE 3 Shot Media D10 (μm) D50 (μm) D90 (μm) Example 1 52 83 142Example 1A 77 118 184 Example 1B 93 129 178 Sinto Micro shot 100 132 183SBM-100C Sinto AMO 68 92 134 BEAD ™ AM-100 Saint Gobain 79 105 148Microblast ™ B120

The elastic modulus of the particles of Example 1 was next determinedusing a nano instrumented indentation tester (IIT). Particle specimenswere prepared by mixing the particles with epoxy resin, curing, and thenpolishing the mix to expose particle cross sections. Four particles oneach sample were selected for testing. The position of the indentationswithin the sample was placed so as to avoid being too close to the edgeof a particle, but were not otherwise particularly selected with anyother criterion. Each test was done with 20 load increments, to amaximum of 20 mN, and then 20 unloading decrements using a diamondBerkovich (triangular pyramid) indenter. The indentation data wasanalyzed using the conventional “Oliver and Pharr” technique. The datawas corrected for initial penetration of the indenter, compliance of theload frame, and area function of the indenter used. The instrument wascalibrated with instrumentation for load and displacement traceable tonational standards of these quantities. Example 1 was determined to havean elastic modulus (assuming a Poisson's ratio of 0.3) of 246 GPa±44.58.

In general, the workpieces that may be employed herein preferablyinclude metal type workpieces that have a HV value of 500-1000. Suchworkpieces may be in a variety of geometrical forms, including but notlimiting to metallic sheet, coils, springs, metal forging or tubes.Accordingly, it is contemplated herein that the shot peening methodutilizing the particles identified herein may be applied to anyworkpiece where shot peening is utilized for any purpose, includingenhancement of general wear characteristics.

For evaluation purposes, SAE 1070 steel Almen A (76 mm×19 mm×1.295±0.025mm thick) strips and Almen N (76 mm×18 mm×0.785±0.025 mm thick) stripswith hardness 472 Vickers and a surface roughness average (Ra) of 0.106micrometers were selected as workpieces. Almen strips are used in theart to quantify the intensity of a shot peening process. Compressivestress induced by the peening operation causes the strip to deform intoan arch, of which the point of maximum curvature is measured using agage specifically designed for the measurement. Arch heights fromsuccessive Almen strips produced under set peening parameters areplotted as a function of time to determine the peening intensitysaturation. Peening intensity saturation is defined as the first pointbeyond which the arc height increases by 10 percent or less when thepeening time is doubled.

Peening intensity saturation was therefore determined by measuring Almenstrip arch heights after peening increments of 5, 10, 20 and 40 seconds.Measurements were taken using a calibrated Electronics Inc. AdvancedAlmen Gage. Peening Intensity Saturation was calculated using SaturationCurve Solver software, Release 9. Post peening maximum subsurfaceresidual compressive stress was determined by X-ray Diffraction (XRD) byprofiling at 12.7, 25.4, 50.8, 101.6, and 127.0 micrometers. XRD peakwidth was converted to hardness for each profile.

Surface roughness measurements were taken using a Mitutoyo SJ-210instrument.

The workpiece post peening residual stress field was determined by X-rayDiffraction (XRD) in accordance with SAE HS-784/2003 by profiling at12.7, 25.4, 50.8, 101.6, and 127.0 micrometers. In measuring residualstress using X-ray diffraction (XRD), the strain in the crystal latticeis measured and the associated residual stress is determined from theelastic constants assuming a linear elastic distortion of theappropriate crystal lattice plane. XRD was performed using a TEC1630with chromium radiation, a peak of 155° 2θ, and a 4 mm diameter roundcollimator. A parabolic fit to the diffraction peak was used, with the kalpha 2 component of the peak subtracted out using SARATec software.Corrections to account for layer removal and exponential penetration ofthe beam (stress gradient effects) were also applied. Hardness wasdetermined by peak width calibrated by microhardness measurements at12.7 and 127 micrometers depth.

Shot peening was carried out using Kelco air blasting equipment usingthe following parameters: pressure of the projection of 0.55 MPa; aventuri nozzle with an exit diameter of 7.9375 mm; a distance to theworkpiece of 101.6 mm; and a mass flow rate of 2.63 kg/minute. Seeagain, FIG. 1.

The results of the peening test are summarized in Table 4 below,comparative peening intensity saturation curves are shown in FIG. 2,comparative residual stress fields are shown in FIG. 3 and correspondingcomparative hardness values in the workpiece are shown in FIG. 4.

TABLE 4 Peening Maximum Intensity residual (Almen A Surface Surfacecompressive Arch Height) Saturation Peening Roughness Hardness stress ofwork- (mils) Time (sec) Time (sec) (Ra/μm) (HV) piece (MPa) Example 15.82 5.72 5 2.372 525 726 10 2.657 526 697 20 2.332 528 702 40 2.234 543685 Example 1A 6.14 16.39 20 1.549 581 982 Example 1B 8.56 14.48 203.261 586 686 Sinto 4.95 9.02 5 1.188 514 705 SBM-100C 10 1.666 514 63520 1.66 524 684 40 1.516 524 684 Sinto 3.5 5.19 5 2.168 526 655 AM-10010 2.058 532 662 20 2.019 538 643 40 1.980 532 632

From Table 4 it can be seen that while the surface roughness of theSinto SBM-100C is lower than that of the Example 1, the maximum residualcompressive stress is also lower with nearly equivalent surfacehardness. This is also reflected in the comparison of the peeningintensity at saturation (Almen intensity (arch height)) versus thepeening saturation time. Further, the longer saturation time of 9.02seconds for Sinto SBM-100C versus that of Example 1 (5.82 seconds) andthe lower maximum residual compressive stress of the commercial SintoSBM-100C near the peening intensity saturation time (635 MPa at 10seconds) versus that of Example 1 (726 MPa at 5 seconds) infers that alower projection pressure could be used with the alloys disclosed hereinfor an equivalent maximum residual stress to that of Sinto SBM-100Cthereby reducing cost. The same conclusion can be drawn when comparingmaximum compressive stress of the commercial Sinto AM-100 near thepeening saturation time (655 MPa at 5 seconds) versus that of InventiveExample 1 (726 MPa at 5 seconds). This is also reflected in FIG. 3, aplot of the XRD through thickness residual stress field profile at nearpeening intensity saturation. FIG. 4 is a plot of the through thicknesshardness profile at near peening intensity, and provides evidence thatExample 1 work hardens the surface in the same manner as Sinto SBM-100Cand Sinto AM-100.

From Table 4 it should again be noted that when comparing to thecommercially available Sinto product, the comparisons are consideredmost relevant at that point where Almen saturation has been achieved.Accordingly, it can be seen that the surface roughness of Example 1A at20 seconds peeing time is lower than that of the Sinto SBM-100C at 10seconds peening time, a higher maximum residual compressive stress isachieved for Example 1A versus that of the Sinto SBM-100C. Further,since the peening intensity is higher for Example 1A than that of theSinto SBM-100C, as shown in Table 4, a deeper residual compressivestress is implied. This is confirmed upon review of the XRD profileresidual stress field, FIG. 4A, and hardness plots, FIG. 4B at nearsaturation for both the Example 1A and the Sinto SBM-100C. In addition,the peening intensity saturation curve for Example 1A and the SintonSBM-100C is provided in FIG. 4C.

From Table 4 it can further be seen that the surface roughness of SintoSBM-100 after 10 seconds peeing time (after Almen saturation has againbeen achieved) is lower than that of Example 1B after 20 seconds (afterAlmen saturation has been achieved). However, a near equivalent maximumresidual compressive stress is achieved for Example 1B versus that ofthe Sinto SBM-100C. Further, since the peening intensity is higher forExample 1B than that of the Sinto SBM-100C with near equivalent maximumresidual compress stress, as shown in Table 4, a deeper residualcompressive stress is implied. This is confirmed upon review of the XRDprofile residual stress field, FIG. 4D, and hardness plots, FIG. 4E atnear saturation for both the Example 1B and the Sinto SBM-100C. Inaddition, the peening intensity saturation curve for Example 1B and theSinton SBM-100C is provided in FIG. 4F.

Example 2

In Example 2, the particles of Example 1 and Example 1B were tested forcomparison with a commercial zirconia ceramic peening media, SaintGobain Microblast® B120, having a specific gravity of 3.8 g/cm³, ahardness of 692 Vickers and a particle size distribution of D10 equaling79 micrometers, D50 equaling 105 micrometers, and D90 equaling 148micrometers was used. SAE 1070 steel Almen N (76 mm×18 mm×0.785±0.025 mmthick) strips with a hardness of 472 Vickers and a surface roughnessaverage (Ra) of 0.106 micrometers were selected as the workpiece. Shotpeening was carried out using Kelco air blasting equipment using thefollowing parameters: pressure of the projection of 0.28 MPa; a venturinozzle with an exit diameter of 7.9375 mm; a distance to the workpieceof 101.6 mm; and a mass flow rate of 2.63 kg/minute. Peening intensitysaturation was determined by measuring Almen strip arch heights afterpeening increments of 5, 10, 20 and 40 seconds. Measurements were takenusing a calibrated Electronics Inc. Advanced Almen Gage. PeeningIntensity Saturation was calculated using Saturation Curve Solversoftware, Release 9. Post peening maximum subsurface residualcompressive stress was determined by X-ray Diffraction (XRD) byprofiling at 12.7, 25.4, 50.8, 101.6, and 127.0 micrometers. XRD peakwidth was converted to hardness for each profile. The results of thepeening test are summarized in Table 5. For Example 1, comparativepeening intensity saturation curves are shown in FIG. 5, comparativeresidual stress fields are shown in FIG. 6 and corresponding comparativehardnesses are shown in FIG. 7. For Example 1B, comparative peeningsaturation curves are shown in FIG. 11, comparative residual stressfields are shown in FIG. 12 and corresponding comparative hardnesses areshown in FIG. 13.

TABLE 5 Peening Maximum Intensity residual (Almen A Surface Surfacecompressive Arch Height) Saturation Peening Roughness Hardness stress ofwork- (mils) Time (sec.) Time (sec.) (Ra/μm) (HV) piece (MPa) Example 13.14 6.17 5 1.524 509 669 10 1.42 515 643 20 1.485 516 587 40 1.679 527534 Example 1B 7.03 16.35 20 1.341 540 671 Saint Gobain 3.09 13.42 50.931 500 679 B120 10 0.872 497 701 20 0.903 505 612 40 0.894 513 555

From Table 5 it can be seen that while the surface roughness of theSaint Gobain B120 is lower than that of the Example 1, near equivalentsurface hardness and maximum residual compressive stress is achieved forExample 1 versus that of the Saint Gobain B120. Further, since thepeening intensity is higher for Example 1 than that of the Saint GobainB120 with near equivalent maximum residual compress stress, as shown inTable 5, a deeper residual compressive stress is implied. This isconfirmed upon review of the XRD profile residual stress field, FIG. 6,and hardness plots, FIG. 7 at saturation for both the Example 1 and theSaint Gobain® B 120.

From Table 5, it can also be seen that while the surface roughness ofSaint Gobain B120 is lower than that of the Example 1B, near equivalentmaximum residual compressive stress is achieved for Example 1B versusthat of the Saint Gobain B120. Further, since the peening intensity ishigher for Example 1B than that of the Saint Gobain B120 with nearequivalent maximum residual compress stress, as shown in Table 5, adeeper residual compressive stress is implied. This is confirmed uponreview of the XRD profile residual stress field, FIG. 12, and hardnessplots, FIG. 13 at saturation for both the Example 1B and the SaintGobain B120.

Example 3

In Example 3, the durability of the Example 1 and 1B, sieved to removefines less than 75 micrometers, was tested in comparison to thedurability of Sinto Microshot SBM-100, Sinto AMO Bead AM-100 and SaintGobain Microblast® B120. Shot peening was carried out using Kelco airblasting equipment using the following parameters: pressure of theprojection: 0.55 MPa; a venturi nozzle with an exit diameter of 7.9375mm; a distance to the workpiece of 101.6 mm; and a mass flow rate of2.63 kg/minute. To test peening media durability, preferably, oneemploys a 6.35 mm thick Hadfield Manganese workpiece peened over aperiod of time using pre-conditioned media. A Hadfield Manganeseworkpiece is made by alloying steel containing 0.8-1.25 wt. % carbonwith 11-15 wt. % manganese. The workpiece has an ultimate tensilestrength of 120,000 psi-140,000 psi, a yield strength of 65,000psi-85,000 psi. The workpiece will also preferably have an initial HBhardness value of 180-245 to a HB hardness value of >500 in the workhardened state. Pre-conditioning is performed by peening the workpiecewith each media for 15 minutes prior testing the media. Durability wasevaluated by weighing the media below 75 micrometers at peeningincrements of 6, 12, 18, and 24 hours. This was done by removing a 100 gsample of shot at the designated time interval, sieving the 100 g mass,and weighing the fraction of particles less than 75 micrometers indiameter to identify the weight percent of fractured material. Theresults of this testing are shown in Table 6.

TABLE 6 Mass Time Loss (hrs.) (%) Inventive Example 1 1 0.30 6 2.00 125.00 18 6.20 Inventive Example 1 0.7 1B 6 3.1 12 3.8 18 5.2 Sinto AM-1006 17.70 12 27.30 18 32.60 Sinto SBM-100C 6 8.00 12 36.00 18 57.00 SaintGobain B120 1 20.00 6 98.00 18 N/A

From Table 6 it is observed that Example 1 and 1B has superiordurability in comparison to all of the commercial samples and inparticular in comparison to the Saint Gobain Microblast® B120. Mass lossfor Example 1 and 1B compared to Sinto AM-100 and Sinto SBM-100C aredepicted graphically in FIG. 8. Mass loss for Example 1 and 1B (comparedto Saint Gobain Microblast® B120) is depicted graphically in FIG. 9.

Accordingly, it may be appreciated that the data above identifies thatthe shot peening metal alloy particles herein may be more broadlyunderstood and defined as particles which, when projected at a pressureof 0.55 MPa, to a preconditioned steel workpiece (i.e. a workpiece thatis peened prior to durability testing), indicate a mass loss (fractionof particles less than 75 μm), after a time period of 18 hours, of lessthan or equal to 20.0%, or ≦19.0%, or ≦18.0%, or ≦17.0%, or ≦16.0%, or≦15.0%, or ≦14.0%, or ≦13.0%, or ≦12.0%, or ≦11.0%, or ≦10.0%, or ≦9.0%,or ≦8.0%, or ≦7.0%. In addition, the particles are such that under theabove identified testing conditions they indicate a mass loss (fractionof particles less than 75 μm) after a time period of up to 12 hours, of≦5.0%. Furthermore, the particles are such that under the aboveidentified testing conditions they indicate a mass loss (fraction ofparticles less than 75 μm) after a time period of up to 6 hours of≦4.0%. Finally, the particles are such that under the above identifiedtesting conditions they indicate a mass loss (fraction of particles lessthan 75 μm) of less ≦1.0%.

As can be seen from the above, the present invention provides animprovement in the use metallic particles that are iron based andcombine relatively high hardness and durability when used as a metalabrasive, as, e.g., in shot-peening processes. The benefits include butare not limited to improvements in the properties that may be realizedin an impacted workpiece as well as increased longevity in the metalparticles employed due to the particles strength and durability as notedherein.

Example 4

In Example 4, the durability of the Example 1, sieved to remove finesless than 75 micrometers, was tested in comparison to the durability ofSaint Gobain Microblast® B120 at a lower projection pressure than thatof Example 3. Shot peening was carried out using Kelco air blastingequipment using the following parameters: pressure of the projection:0.28 MPa; a venturi nozzle with an exit diameter of 7.9375 mm; adistance to the workpiece of 101.6 mm; and a mass flow rate of 2.63kg/minute. To test peening media durability a 6.35 mm thick HadfieldManganese workpiece (described above) peened over a period of time usingpre-conditioned media. Pre-conditioning was performed by peening theworkpiece with each media for 15 minutes prior testing the media.Durability was evaluated by weighing the media below 75 micrometers atpeening increments of 1, 3, and 6 hours. This was done by removing a 100g sample of shot at the designated time interval, sieving the 100 gmass, and weighing the fraction of particles less than 75 micrometers indiameter to identify the weight percent of fractured material.Accordingly, it may now be appreciated that one characteristic of theshot peening media herein is that the metal alloy particles, whenprojected at a pressure of 0.28 MPa at a Hadfeld Manganese workpiece,

TABLE 7 Mass Time Loss (hrs.) (%) Inventive Example 1 1 0 3 0 6 1.0Saint Gobain B120 1 3.6 3 7.8 6 17.3

From Table 7 it is observed that Example 1 has superior durability incomparison to the Saint Gobain Microblast® B120. Mass loss for Example 1according to the above, as compared to Saint Gobain Microblast® B120, isdepicted graphically in FIG. 10.

Accordingly, it may be appreciated that the data above identifies thatthe shot peening metal alloy particles herein may be more broadlyunderstood and defined as particles which, when projected at a pressureof 0.28 MPa, to a preconditioned steel workpiece (i.e. a workpiece thatis peened prior to durability testing), indicate a mass loss (fractionof particles less than 75 μm), after a time period of 6 hours, of lessthan or equal to 15.0% or lower, such as ≦14.0%, or ≦13.0%, or ≦12.0%,or ≦11.0%, or ≦10.0%, or ≦9.0%, or ≦8.0%, or ≦7.0%, or 6.0%, or ≦5.0%,or ≦4.0%, or ≦3.0%, or ≦2.0%, or ≦1.0%. In addition, the particles aresuch that under the above identified testing conditions they indicate amass loss (fraction of particles less than 75 μm) after a time period ofup to 3 hours, of ≦5.0%, or ≦4.0%, or ≦3.0%, or ≦2.0%, or ≦1.0%.Furthermore, the particles are such that under the above identifiedtesting conditions they indicate a mass loss (fraction of particles lessthan 75 μm) after a time period of up to 1 hour of ≦3.0%, or ≦2.0%, or≦1.0%.

Applications for the shot peening particles herein include, but are notlimited to, gear parts, cams and camshafts, clutch springs, coilsprings, connecting rods, crankshafts, gearwheels, leaf and suspensionsprings, threads, rock drills, and turbine blades. One particularlyuseful application has been determined to include engine valve springs,which operate to maintain the engine valve springs closed against theirseating until a cam opens such valve to release pressure. Such enginevalve springs particularly include engine valve springs that have arelatively high hardness, such as a chromium-silicon type valve springalloy, which has a hardness (ASTM A877) of HRC 48-55.

1. A method of shot peening a workpiece comprising projecting metalalloy particles at a workpiece wherein said metal alloy particlescomprises Fe in combination with B, C, Cr and Nb, wherein the Fe ispresent at a level of greater than 50.0 atomic percent wherein saidmetal alloy has a Vickers Hardness (HV) of at least 1150 and an elasticmodulus of greater than 200 GPa.
 2. The method of claim 1 wherein saidmetal alloy has a HV of 1150-1400.
 3. The method of claim 1 wherein saidmetal alloy has an elastic modulus in the range of greater than 200 GPato 350 GPa.
 4. The method of claim 1 wherein said metal alloy comprises:Fe: 59.0-64.0 at. % B: 17.5-18.8 at. % C: 4.4-5.1 at. % Cr: 12.7-13.1at. % Nb: 1.4-1.7 at. %.
 5. The method of claim 1 wherein 95% of theparticles have a size in the range of 40 μm to 250 μm.
 6. The method ofclaim 1 wherein the particles have the following particle sizedistribution: D10: 50 μm D50: 80 μm D90: 150 μm.
 7. The method of claim1 wherein the weight fraction of particles of less than 75 microns isless than or equal to 7.0%, after projection of said particles onto saidworkpiece.
 8. The method of claim 1 wherein the shot is projected at ametal substance having a HV in the range of 500 to
 1000. 9. The methodof claim 1 wherein said alloy includes α-Fe and/or γ-Fe and one or moreof the following: (1) complex borides; (2) complex carbides; or (3)borocarbides.
 10. The method of claim 1 wherein said alloy comprises:Fe: 58.0-65.0 at. % B: 14.0-19.0 at. % C: 4.4-5.5 at. % Cr: 7.0-13.5 at.% Nb: 1.4-3.5 at. %.
 11. The method of claim 1 wherein the particleshave the following particle size distribution: D10: 50-100 μm D50:100-150 μm D90: 150-200 μm
 12. The method of claim 1 wherein saidparticles, when projected at a pressure of 0.55 MPa, to a preconditionedsteel workpiece, indicate a mass loss associated with the fraction ofparticles less than 75.0 μm, after a time period of 18.0 hours, of lessthan or equal to 20.0%.
 13. The method of claim 1 wherein saidparticles, when projected at a pressure of 0.28 MPa, to a preconditionedsteel workpiece, indicate a mass loss associated with the fraction ofparticles of less than 75.0 μm, after a time period of 6 hours, of lessthan or equal to 15.0%.
 14. The method of claim 1 wherein said workpiececomprises one of a gear part, cam, camshaft, clutch spring, coil spring,connecting rod, crankshaft, gearwheel, leaf spring, suspension spring,threads, rock drill, turbine blades or engine valve spring.
 15. Shotpeening material comprising metal alloy particles containing Fe incombination with B, C, Cr and Nb, wherein the Fe is present at a levelof greater than 50.0 atomic percent wherein said metal alloy has aVickers Hardness (HV) of at least 1150 and an elastic modulus of greaterthan 200 GPa.
 16. Shot peening material of claim 15 wherein said metalalloy has a HV of 1150-1400.
 17. Shot peening material of claim 15wherein said metal alloy has an elastic modulus in the range of greaterthan 200 GPa to 350 GPa.
 18. Shot peening material of claim 15 whereinsaid metal alloy comprises: Fe: 59.0-64.0 at. % B: 17.5-18.8 at. % C:4.4-5.1 at. % Cr: 12.7-13.1 at. % Nb: 1.4-1.7 at. %.
 19. Shot peeningmaterial of claim 15 wherein 95% of the particles have a size in therange of 40 μm to 250 μm.
 20. The shot peening material of claim 15wherein said alloy comprises: Fe: 58.0-65.0 at. % B: 14.0-19.0 at. % C:4.4-5.5 at. % Cr: 7.0-13.5 at. % Nb: 1.4-3.5 at. %.
 21. The shot peeningmaterial of claim 15 wherein the particles have the following particlesize distribution: D10: 50-100 μm D50: 100-150 μm D90: 150-200 μm.